1. Field of the Invention
The present invention is directed generally to a method and apparatus for providing a voltage to a communication device and, more particularly, to a portable communication device having improved back-lighting and key-press noise margin.
2. Description of the Background
Portable communication devices, such as portable telephones, often include, for example, a main module, which main module often includes a personal digital assistant mainboard that performs the functions of portable telephone call receipt, call decoding, call sending, and encoding, as well as personal digital assistant functions, such as address books and paging. Portable communication devices additionally often include a flip module. The flip module generally opens outward from the main module, and regularly includes thereon a series of back-lit keys that, when pressed, cause the performance of various functions by the mainboard. The interface between the flip module and the main module may be, for example, a two-wire interface.
The two wire interface performs the function of carrying the voltage necessary to power the light-emitting diodes (LEDs) that may perform the backlighting on the flip module, and additionally performs the function of carrying the signal signifying the pressing of a particular key on the flip module to the mainboard. Upon receipt of this signal, the mainboard then carries out at least one of a series of functions, dependant upon which key or keys were pressed.
Thus, voltage has historically been placed on the two-wire interface in order to send voltage to, and receive voltage from, the flip module to perform the dual flip module functions of reading key presses and back-lighting. In general, a first voltage has been used to read key presses. This voltage is hereinafter referred to as the xe2x80x9ckey press voltagexe2x80x9d. A second, larger voltage has been used to perform backlighting. This is referred to as the xe2x80x9cbacklight voltage.xe2x80x9d
Due to the fact that the two voltages discussed hereinabove share the same 2-wire interface, only one voltage occupies the interface at any given time, in order to maintain separate operation of the key press and backlight functions. The prior art has thus sequenced the two voltages, connecting the backlight voltage to the interface by an electrical switch, and making the key press of high source impedance. When the switch is off, the interface defaults to the key press voltage. When the switch is on, the backlight voltage is on the interface, overriding the high source impedance of the key press voltage. In operation, the backlight voltage has historically been active substantially continuosly, except for periodic intervals of key press detection.
In order to prevent undesirable interaction between the key press circuitry and the LEDs, a switch is employed on the flip module to connect the LEDs to the 2-wire interface only upon receipt of the backlight voltage from the mainboard. Due to the fact that a low-cost Darlington switch topology used in the prior art had an inherently low threshold precision, the backlight voltage has historically been substantially higher than the maximum key press voltage to ensure proper operation of the LED switch. Backlight voltage is thus higher than key press voltage, to allow the LED switch to distinguish between the two voltages. In general, backlight voltage has historically been approximately 3.3 volts, and the key press voltage has been in the range of between 0 and approximately 1.6 volts.
Each key press has generally corresponded to a different voltage at the interface, and, thus the sum of the voltage ranges of all keys must be present within the 1.5 Volt range available as set forth hereinabove. Therefore, the probability of misreading a key has been unacceptably high, due to the very small range available to each key when a small voltage range, such as 1.5V, is used (if there are 20 keys, each key has a range of only 1.5V/20 keys, or 0.075V per key). Further, the key presses are much more susceptible to interference and noise, due to a low signal to noise ratio, than the key presses would be if a wider range was available to each key.
Further, a larger reduction in voltage on the interface, to insure that the LEDs were not seen by the mainboard, was often necessary due to the use of Darlington transistor switching to turn the LEDs off and on. Transistor switches preferably have well-defined on and off states. However, Darlington transistor circuits frequently suffer from uncertainty between on and off states, and thus large voltage changes are necessary to the LEDs on the flip module to insure that the Darlington switch is in the desired state.
Therefore, the need exists for a communication device that uses a two-wire voltage interface, but that additionally provides an improved signal to noise ratio for key presses, better isolates the backlighting voltage from the key press voltage, and that makes a larger voltage range available for both back-lighting and key press reading of the communication device.
The present invention is directed to a mobile communication device. The mobile communication device includes a main module having a call receiver and a call sender, a keyboard module resident on a flip module separate from the main module, which keyboard module includes thereon a plurality of marked keys backlit by at least one light, a central processing unit which controls the activation of the at least one light, a two wire interface having a first voltage line and a second voltage line interconnecting the flip module and the main module, and a voltage switch which places a first positive voltage between the first voltage line and the second voltage line to allow for a reading of a pressed at least one of the plurality of keys by the central processing unit, and which places a first negative voltage between the first voltage line and the second voltage line of to activate the at least one light according to a command from the CPU. The main module may additionally include a main board that includes thereon the call sender and call receiver. The voltage switch preferably includes a keyboard flip module switch and a main board switch connected by said two wire interface.
The present invention also includes a method of reading key presses and backlighting a mobile communication device. The method includes providing a main module having a call receiver and a call sender, providing a keyboard module resident on a flip module separate from the main module, wherein the keyboard module includes thereon a plurality of marked keys, backlighting the plurality of keys by at least one light, controlling, by a central processing unit, of activation of the at least one light, interconnecting the flip module and the main module using a two wire interface having a first voltage line and a second voltage line, receiving a first switch command from the CPU, switching to a first positive voltage between the first voltage line and the second voltage line according to the receiving of a first switch command, receiving a second switch command from the CPU, switching to a first negative voltage between the first voltage line and the second voltage line according to the receiving of a second switch command, and activating the at least one light upon said switching to a first negative voltage. The method preferably further includes isolating the CPU from the backlighting step.
The present invention solves problems experienced with the prior art because it provides a mobile communication device that uses a two-wire voltage interface, but that additionally provides an improved signal to noise ratio for key presses, better isolates backlight voltage from key press voltage, and that makes a larger voltage range available for both back-lighting and key press reading of the communication device. Those and other advantages and benefits of the present invention will become apparent from the detailed description of the invention hereinbelow.